OK, here is my off-the-top-of-my-head description of how to find Pwll Dwfn. I strongly recommend, however, the use of a GPS or someone who's been there before!
From the DYO car park take the obvious path from the campsite up the hill. Go through the kissing gate next to the breeze-block sheep pens (from where the River Haffes is visible to your right), and keep following the path up a steep incline and round a couple of bends, to where there is a tree and a wooden sign with a walker symbol on it. Follow the direction the sign is pointing, which will take you alongside a ramshackle dry stone wall with a wire fence on it. After a couple of hundred metres the obvious path veers R away from the wall up onto the hillside; this bit is quite steep and as it starts to level off (getting close to Tunnel Cave, for those who know where that is) you will see a much less significant path (more a sheep track) on your left that heads up the hill at an angle, heading vaguely S. It is more obvious when the bracken is lower, as at this time of year. (The sharp-eyed may spot that there are two such paths; you want the upper one.) Take this path up and over a crest to where it descends into a dry valley: the entrance to Pwll Dwfn is immediately adjacent to the path, on the R as you walk from DYO, at pretty much the lowest point of the dry valley. There is a (very) poor excuse for a wall around it; it's just a few stones placed in a line so don't waste time looking for a proper wall! As someone has already pointed out, there is a large rectangular-ish block immediately above the drop down into the cave. In all but the driest conditions you can hear water dripping and echoing if you stick your head in, and in doing this you will notice the wear-marks where cavers have slithered down the slightly awkward entrance.
Beware of the fact the fact that the start of the first pitch is almost immediately inside the entrance, although there is room to slide down and sort yourself out before you need to start rigging.
I hasten to point that I accept no responsibility for anyone getting lost as result of reading the above! If you end up in the dry valley and can't find the entrance as described it is most likely that you have taken the lower of the two sheep tracks, so head uphill - the correct path is pretty obvious at the point where it crosses the dry valley.
